System and method for synthesis of speech from provided text

ABSTRACT

A system and method are presented for the synthesis of speech from provided text. Particularly, the generation of parameters within the system is performed as a continuous approximation in order to mimic the natural flow of speech as opposed to a step-wise approximation of the feature stream. Provided text may be partitioned and parameters generated using a speech model. The generated parameters from the speech model may then be used in a post-processing step to obtain a new set of parameters for application in speech synthesis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/596,628, filed on Jan. 14, 2015, which claims priority and benefit ofU.S. Provisional Application No. 61/927,152, filed on Jan. 14, 2014, thecontents of both of which are incorporated herein by reference.

BACKGROUND

The present invention generally relates to telecommunications systemsand methods, as well as speech synthesis. More particularly, the presentinvention pertains to synthesizing speech from provided text usingparameter generation.

SUMMARY

A system and method are presented for the synthesis of speech fromprovided text. Particularly, the generation of parameters within thesystem is performed as a continuous approximation in order to mimic thenatural flow of speech as opposed to a step-wise approximation of theparameter stream. Provided text may be partitioned and parametersgenerated using a speech model. The generated parameters from the speechmodel may then be used in a post-processing step to obtain a new set ofparameters for application in speech synthesis.

In one embodiment, a system is presented for synthesizing speech forprovided text comprising: means for generating context labels for saidprovided text; means for generating a set of parameters for the contextlabels generated for said provided text using a speech model; means forprocessing said generated set of parameters, wherein said means forprocessing is capable of variance scaling; and means for synthesizingspeech for said provided text, wherein said means for synthesizingspeech is capable of applying the processed set of parameters tosynthesizing speech.

In another embodiment, a method for generating parameters, using acontinuous feature stream, for provided text for use in speechsynthesis, is presented, comprising the steps of: partitioning saidprovided text into a sequence of phrases; generating parameters for saidsequence of phrases using a speech model; and processing the generatedparameters to obtain an other set of parameters, wherein said other setof parameters are capable of use in speech synthesis for provided text.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an embodiment of a system forsynthesizing speech.

FIG. 2 is a diagram illustrating a modified embodiment of a system forsynthesizing speech.

FIG. 3 is a flowchart illustrating an embodiment of parametergeneration.

FIG. 4 is a diagram illustrating an embodiment of a generated parameter.

FIG. 5 is a flowchart illustrating an embodiment of a process for f0parameter generation.

FIG. 6 is a flowchart illustrating an embodiment of a process for MCEPsgeneration.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described embodiments, and any further applications of theprinciples of the invention as described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

In a traditional text-to-speech (TTS) system, written language, or text,may be automatically converted into linguistic specification. Thelinguistic specification indexes the stored form of a speech corpus, orthe model of speech corpus, to generate speech waveform. A statisticalparametric speech system does not store any speech itself, but the modelof speech instead. The model of the speech corpus and the output of thelinguistic analysis may be used to estimate a set of parameters whichare used to synthesize the output speech. The model of the speech corpusincludes mean and covariance of the probability function that the speechparameters fit. The retrieved model may generate spectral parameters,such as fundamental frequency (f0) and mel-cepstral (MCEPs), torepresent the speech signal. These parameters, however, are for a fixedframe rate and are derived from a state machine. A step-wiseapproximation of the parameter stream results, which does not mimic thenatural flow of speech. Natural speech is continuous and not step-wise.In one embodiment, a system and method are disclosed that converts thestep-wise approximation from the models to a continuous stream in orderto mimic the natural flow of speech.

FIG. 1 is a diagram illustrating an embodiment of a traditional systemfor synthesizing speech, indicated generally at 100. The basiccomponents of a speech synthesis system may include a training module105, which may comprise a speech corpus 106, linguistic specifications107, and a parameterization module 108, and a synthesizing module 110,which may comprise text 111, context labels 112, a statisticalparametric model 113, and a speech synthesis module 114.

The training module 105 may be used to train the statistical parametricmodel 113. The training module 105 may comprise a speech corpus 106,linguistic specifications 107, and a parameterization module 108. Thespeech corpus 106 may be converted into the linguistic specifications107. The speech corpus may comprise written language or text that hasbeen chosen to cover sounds made in a language in the context ofsyllables and words that make up the vocabulary of the language. Thelinguistic specification 107 indexes the stored form of speech corpus orthe model of speech corpus to generate speech waveform. Speech itself isnot stored, but the model of speech is stored. The model includes meanand the covariance of the probability function that the speechparameters fit.

The synthesizing module 110 may store the model of speech and generatespeech. The synthesizing module 110 may comprise text 111, contextlabels 112, a statistical parametric model 113, and a speech synthesismodule 114. Context labels 112 represent the contextual information inthe text 111 which can be of a varied granularity, such as informationabout surrounding sounds, surrounding words, surrounding phrases, etc.The context labels 112 may be generated for the provided text from alanguage model. The statistical parametric model 113 may include meanand covariance of the probability function that the speech parametersfit.

The speech synthesis module 114 receives the speech parameters for thetext 111 and transforms the parameters into synthesized speech. This canbe done using standard methods to transform spectral information intotime domain signals, such as a mel log spectrum approximation (MLSA)filter.

FIG. 2 is a diagram illustrating a modified embodiment of a system forsynthesizing speech using parameter generation, indicated generally at200. The basic components of a system may include similar components tothose in FIG. 1, with the addition of a parameter generation module 205.In a statistical parametric speech synthesis system, the speech signalis represented as a set of parameters at some fixed frame rate. Theparameter generation module 205 receives the audio signal from thestatistical parameter model 113 and transforms it. In an embodiment, theaudio signal in the time domain has been mathematically transformed toanother domain, such as the spectral domain, for more efficientprocessing. The spectral information is then stored as the form offrequency coefficients, such as f0 and MCEPs to represent the speechsignal. Parameter generation is such that it has an indexed speech modelas input and the spectral parameters as output. In one embodiment,Hidden Markov Model (HMM) techniques are used. The model 113 includesnot only the statistical distribution of parameters, also called staticcoefficients, but also their rate of change. The rate of change may bedescribed as having first-order derivatives called delta coefficientsand second-order derivatives referred to as deltadelta coefficients. Thethree types of parameters are stacked together into a single observationvector for the model. The process of generating parameters is describedin greater detail below.

In the traditional statistical model of the parameters, only the meanand the variance of the parameter are considered. The mean parameter isused for each state to generate parameters. This generates piecewiseconstant parameter trajectories, which change value abruptly at eachstate transition, and is contrary to the behavior of natural sound.Further, the statistical properties of the static coefficient are onlyconsidered and not the speed with which the parameters change value.Thus, the statistical properties of the first- and second-orderderivatives must be considered, as in the modified embodiment describedin FIG. 2.

Maximum likelihood parameter generation (MLPG) is a method thatconsiders the statistical properties of static coefficients and thederivatives. However, this method has a great computational cost thatincreases with the length of the sequence and thus is impractical toimplement in a real-time system. A more efficient method is describedbelow which generates parameters based on linguistic segments instead ofwhole text message. A linguistic segment may refer to any group of wordsor sentences which can be separated by context label “pause” in a TTSsystem.

FIG. 3 is a flowchart illustrating an embodiment of generating parametertrajectories, indicated generally at 300. Parameter trajectories aregenerated based on linguistic segments instead of whole text message.Prior to parameter generation, a state sequence may be chosen using aduration model present in the statistical parameter model 113. Thisdetermines how many frames will be generated from each state in thestatistical parameter model. As hypothesized by the parameter generationmodule, the parameters do not vary while in the same state. Thistrajectory will result in a poor quality speech signal. However, if asmoother trajectory is estimated using information from delta anddelta-delta parameters, the speech synthesis output is more natural andintelligible.

In operation 305, the state sequence is chosen. For example, the statesequence may be chosen using the statistical parameter model 113, whichdetermines how many frames will be generated from each state in themodel 113. Control passes to operation 310 and process 300 continues.

In operation 310, segments are partitioned. In one embodiment, thesegment partition is defined as a sequence of states encompassed by thepause model. Control is passed to at least one of operations 315 a and315 b and process 300 continues.

In operations 315 a and 315 b, spectral parameters are generated. Thespectral parameters represent the speech signal and comprise at leastone of the fundamental frequency 315 a and MCEPs, 315 b. These processesare described in greater detail below in FIGS. 5 and 6. Control ispassed to operation 320 and process 300 continues.

In operation 320, the parameter trajectory is created. For example, theparameter trajectory may be created by concatenating each parameterstream across all states along the time domain. In effect each dimensionin the parametric model will have a trajectory. An illustration of aparameter trajectory creation for one such dimension is providedgenerally in FIG. 4. FIG. 4 (copied from: KING, Simon, “A beginners'guide to statistical parametric speech synthesis” The Centre for SpeechTechnology Research, University of Edinburgh, UK, 24 Jun. 2010, page 9)is a generalized embodiment of a trajectory from MLPG that has beensmoothed.

FIG. 5 is a flowchart illustrating an embodiment of a process forfundamental spectral parameter generation, indicated generally at 500.The process may occur in the parameter generation module 205 (FIG. 2)after the input text is split into linguistic segments. Parameters arepredicted for each segment.

In operation 505, the frame is incremented. For example, a frame may beexamined for linguistic segments which may contain several voicedsegments. The parameter stream may be based on frame units such that i=1represents the first frame, i=2 represents the second frame, etc. Forframe incrementing, the value for “i” is increased by a desiredinterval. In an embodiment, the value for “i” may be increased by 1 eachtime. Control is passed to operation 510 and the process 500 continues.

In operation 510, it is determined whether or not linguistic segmentsare present in the signal. If it is determined those linguistic segmentsare present, control is passed to operation 515 and process 500continues. If it is determined that linguistic segments are not present,control is passed to operation 525 and the process 500 continues.

The determination in operation 510 may be made based on any suitablecriteria. In one embodiment, the segment partition of the linguisticsegments is defined as a sequence of states encompassed by the pausemodel.

In operation 515, a global variance adjustment is performed. Forexample, the global variance may be used to adjust the variance of thelinguistic segment. The f0 trajectory may tend to have a smaller dynamicrange compared to natural sound due to the use of the mean of the staticcoefficient and the delta coefficient in parameter generation. Variancescaling may expand the dynamic range of the f0 trajectory so that thesynthesized signal sounds livelier. Control is passed to operation 520and process 500 continues.

In operation 520, a conversion to the linear frequency domain isperformed on the fundamental frequency from the log domain and theprocess 500 ends.

In operation 525, it is determined whether or not the voicing hasstarted. If it is determined that the voicing has not started, controlis passed to operation 530 and the process 500 continues. If it isdetermined that voicing has started, control is passed to operation 535and the process 500 continues.

The determination in operation 525 may be based on any suitablecriteria. In an embodiment, when the f0 model predicts valid values forf0, the segment is deemed a voiced segment and when the f0 modelpredicts zeros, the segment is deemed an unvoiced segment.

In operation 530, the frame has been determined to be unvoiced. Thespectral parameter for that frame is 0 such that f0(i)=0. Control ispassed back to operation 505 and the process 500 continues.

In operation 535, the frame has been determined to be voiced and it isfurther determined whether or not the voicing is in the first frame. Ifit is determined that the voicing is in the first frame, control ispassed to operation 540 and process 500 continues. If it is determinedthat the voicing is not in the first frame, control is passed tooperation 545 and process 500 continues.

The determination in operation 535 may be based on any suitablecriteria. In one embodiment it is based on predicted f0 values and inanother embodiment it could be based on a specific model to predictvoicing.

In operation 540, the spectral parameter for the first frame is the meanof the segment such that f0(i)=f0_mean(i). Control is passed back tooperation 505 and the process 500 continues.

In operation 545, it is determined whether or not the delta value needsto be adjusted. If it is determined that the delta value needs adjusted,control is passed to operation 550 and the process 500 continues. If itis determined that the delta value does not need adjusted, control ispassed to operation 555 and the process 500 continues.

The determination in operation 545 may be based on any suitablecriteria. For example, an adjustment may need to be made in order tocontrol the parameter change for each frame to a desired level.

In operation 550, the delta is clamped. The f0_deltaMean(i) may berepresented as f0_new_deltaMean(i) after clamping. If clamping has notbeen performed, then the f0_new_deltaMean(i) is equivalent tof0_deltaMean(i). The purpose of clamping the delta is to ensure that theparameter change for each frame is controlled to a desired level. If thechange is too large, and say lasts over several frames, the range of theparameter trajectory will not be in the desired natural sound's range.Control is passed to operation 555 and the process 500 continues.

In operation 555, the value of the current parameter is updated to bethe predicted value plus the value of delta for the parameter such thatf0(i)=f0(i−1)+f0_new_deltaMean(i). This helps the trajectory ramp up ordown as per the model. Control is then passed to operation 560 and theprocess 500 continues.

In operation 560, it is determined whether or not the voice has ended.If it is determined that the voice has not ended, control is passed tooperation 505 and the process 500 continues. If it is determined thatthe voice has ended, control is passed to operation 565 and the process500 continues.

The determination in operation 560 may be determined based on anysuitable criteria. In an embodiment the f0 values becoming zero for anumber of consecutive frames may indicate the voice has ended.

In operation 565, a mean shift is performed. For example, once all ofthe voiced frames, or voiced segments, have ended, the mean of the voicesegment may be adjusted to the desired value. Mean adjustment may alsobring the parameter trajectory come into the desired natural sound'srange. Control is passed to operation 570 and the process 500 continues.

In operation 570, the voice segment is smoothed. For example, thegenerated parameter trajectory may have abruptly changed somewhere,which makes the synthesized speech sound warble and jumpy. Long windowsmoothing can make the f0 trajectory smoother and the synthesized speechsound more natural. Control is passed back to operation 505 and theprocess 500 continues. The process may continuously cycle any number oftimes that are necessary. Each frame may be processed until thelinguistic segment ends, which may contain several voiced segments. Thevariance of the linguistic segment may be adjusted based on globalvariance. Because the mean of static coefficients and delta coefficientsare used in parameter generation, the parameter trajectory may havesmaller dynamic ranges compared to natural sound. A variance scalingmethod may be utilized to expand the dynamic range of the parametertrajectory so that the synthesized signal does not sound muffled. Thespectral parameters may then be converted from the log domain into thelinear domain.

FIG. 6 is a flowchart illustrating an embodiment of MCEPs generation,indicated generally at 600. The process may occur in the parametergeneration module 205 (FIG. 2).

In operation 605, the output parameter value is initialized. In anembodiment, the output parameter may be initialized at time i=0 becausethe output parameter value is dependent on the parameter generated forthe previous frame. Thus, the initial mcep(0)=mcep_mean(1). Control ispassed to operation 610 and the process 600 continues.

In operation 610, the frame is incremented. For example, a frame may beexamined for linguistic segments which may contain several voicedsegments. The parameter stream may be based on frame units such that i=1represents the first frame, i=2 represents the second frame, etc. Forframe incrementing, the value for “i” is increased by a desiredinterval. In an embodiment, the value for “i” may be increased by 1 eachtime. Control is passed to operation 615 and the process 600 continues.

In operation 615, it is determined whether or not the segment is ended.If it is determined that the segment has ended, control is passed tooperation 620 and the process 600 continues. If it is determined thatthe segment has not ended, control is passed to operation 630 and theprocess continues.

The determination in operation 615 is made using information fromlinguistic module as well as existence of pause.

In operation 620, the voice segment is smoothed. For example, thegenerated parameter trajectory may have abruptly changed somewhere,which makes the synthesized speech sound warble and jumpy. Long windowsmoothing can make the trajectory smoother and the synthesized speechsound more natural. Control is passed to operation 625 and the process600 continues.

In operation 625, a global variance adjustment is performed. Forexample, the global variance may be used to adjust the variance of thelinguistic segment. The trajectory may tend to have a smaller dynamicrange compared to natural sound due to the use of the mean of the staticcoefficient and the delta coefficient in parameter generation. Variancescaling may expand the dynamic range of the trajectory so that thesynthesized signal should not sound muffled. The process 600 ends.

In operation 630, it is determined whether or not the voicing hasstarted. If it is determined that the voicing has not started, controlis passed to operation 635 and the process 600 continues. If it isdetermined that voicing has started, control is passed to operation 540and the process 600 continues.

The determination in operation 630 may be made based on any suitablecriteria. In an embodiment, when the f0 model predicts valid values forf0, the segment is deemed a voiced segment and when the f0 modelpredicts zeros, the segment is deemed an unvoiced segment.

In operation 635, the spectral parameter is determined. The spectralparameter for that frame becomes mcep(i)=(mcep(i−1)+mcep_mean(i))/2.Control is passed back to operation 610 and the process 600 continues.

In operation 640, the frame has been determined to be voiced and it isfurther determined whether or not the voice is in the first frame. If itis determined that the voice is in the first frame, control is passedback to operation 635 and process 600 continues. If it is determinedthat the voice is not in the first frame, control is passed to operation645 and process 500 continues.

In operation 645, the voice is not in the first frame and the spectralparameter becomes mcep(i)=(mcep(i−1)+mcep_delta(i)+mcep_mean(i))/2.Control is passed back to operation 610 and process 600 continues. In anembodiment, multiple MCEPs may be present in the system. Process 600 maybe repeated any number of times until all MCEPs have been processed.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allequivalents, changes, and modifications that come within the spirit ofthe invention as described herein and/or by the following claims aredesired to be protected.

Hence, the proper scope of the present invention should be determinedonly by the broadest interpretation of the appended claims so as toencompass all such modifications as well as all relationships equivalentto those illustrated in the drawings and described in the specification.

The invention claimed is:
 1. A method for synthesizing speech from inputtext, the method comprising: generating context labels from the inputtext, the context labels comprising one or more pause labels;partitioning the input text into a plurality of linguistic segments inaccordance with the one or more pause labels; generating, for eachlinguistic segment, a time domain audio signal from the linguisticsegment in accordance with a statistical parameter model; generating,for each linguistic segment, a parameter trajectory from the time domainaudio signal, the parameter trajectory comprising a plurality of framesfor the linguistic segment, each frame comprising a vector ofparameters; smoothing a transition between a first frame and a secondframe of the frames of the parameter trajectory; and synthesizing speechfrom the parameter trajectory; wherein the vector of parameters for eachframe of the parameter trajectory comprises one or more frequencycoefficients, spectral envelope values, delta coefficients, anddelta-delta coefficients, and wherein the smoothing the transitionbetween the first frame and the second frame of the frames of theparameter trajectory comprises clamping at least one delta coefficientof the delta coefficients corresponding to the first frame and thesecond frame.
 2. The method of claim 1, wherein the context labels aregenerated based on linguistic analysis of the input text.
 3. The methodof claim 1, wherein the frames of the parameter trajectory of thelinguistic segment are grouped into a sequence of states, wherein thevectors of parameters for the frames are generated separately for eachstate of the sequence of states.
 4. The method of claim 1, wherein thegenerating the parameter trajectory comprises transforming the timedomain audio signal to a spectral domain.
 5. The method of claim 1,wherein the statistical parameter model is trained by: converting aspeech corpus into a linguistic specification, the speech corpuscovering sounds made in a language and the linguistic specificationindexing the speech corpus to generate a speech waveform based onspectral speech parameters; and generating the statistical parametermodel based on the linguistic specification and a mean and covariance ofa probability function fit by the spectral speech parameters.
 6. Amethod for synthesizing speech from input text, the method comprising:generating context labels from the input text, the context labelscomprising one or more pause labels; partitioning the input text into aplurality of linguistic segments in accordance with the one or morepause labels; generating, for each linguistic segment, a time domainaudio signal from the linguistic segment in accordance with astatistical parameter model; generating, for each linguistic segment, aparameter trajectory from the time domain audio signal, the parametertrajectory comprising a plurality of frames for the linguistic segment,each frame comprising a vector of parameters; smoothing a transitionbetween a first frame and a second frame of the frames of the parametertrajectory; and synthesizing speech from the parameter trajectory;wherein the generating the parameter trajectory for a linguistic segmentcomprises generating a plurality of mel-cepstral coefficients by, foreach frame of the parameter trajectory, where i is an index referring toa current frame: setting a mel-cepstral coefficient of a first frame ofthe parameter trajectory to a mean value of a second frame of theparameter trajectory; determining if the frame is voiced, wherein; ifthe segment is unvoiced, setting the mel-cepstral coefficient of thecurrent frame (mcep(i)) to (mcep(i−1)+mcep_mean(i))/2; if the segment isvoiced and is a first frame, then settingmcep(i)=(mcep(i−1)+mcep_mean(i))/2; and if the segment is voiced and isnot a first frame, then setting mcep(i)=(mcep(i−1)+mcepdelta(i)+mcep_mean(i))/2; determining if the linguistic segment hasended, wherein: when the linguistic segment has ended, removing abruptchanges of the parameter trajectory and adjusting global variance; andwhen the linguistic segment has not ended, incrementing the index i andrepeating for the next frame of the parameter trajectory.
 7. The methodof claim 6, wherein the statistical parameter model is trained by:converting a speech corpus into a linguistic specification, the speechcorpus covering sounds made in a language and the linguisticspecification indexing the speech corpus to generate a speech waveformbased on spectral speech parameters; and generating the statisticalparameter model based on the linguistic specification and a mean andcovariance of a probability function fit by the spectral speechparameters.
 8. The method of claim 6, wherein the context labels aregenerated based on linguistic analysis of the input text.